Although wellbores are normally drilled vertically, it is sometimes necessary or advantageous to drill at an angle from the vertical.

Controlled directional drilling makes is possible to reach subsurface areas laterally remote from the point where the bit enters the earth.

It often involves the use of turbodrills, dyna-drills, whipstocks, or other deflecting rods.

2. the intentional deviation of a wellbore from the path it would naturally take.

This is accomplished through the use of whipstocks, bottomhole assembly (bha) configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors and special bha components and drill bits.

The directional driller also exploits drilling parameters such as weight on bit and rotary speed to deflect the bit away from the axis of the existing wellbore.

In some cases, such as drilling steeply dipping formations or unpredictable deviation in conventional drilling operations, directional-drilling techniques may be employed to ensure that the hole is drilled vertically.

While many techniques can accomplish this, the general concept is simple: point the bit in the direction that one wants to drill.

The most common way is through the use of a bend near the bit in a downhole steerable mud motor.

The bend points the bit in a direction different from the axis of the wellbore when the entire drillstring is not rotating.

By pumping mud through the mud motor, the bit turns while the drillstring does not rotate, allowing the bit to drill in the direction it points.

When a particular wellbore direction is achieved, that direction may be maintained by rotating the entire drillstring (including the bent section) so that the bit does not drill in a single direction off the wellbore axis, but instead sweeps around and its net direction coincides with the existing wellbore. rotary steerable tools allow steering while rotating, usually with higher rates of penetration and ultimately smoother boreholes.

A tool for drilling rock that works by scraping industrial grade diamonds against the bottom of the hole.

The diamonds are embedded into the metal structure (usually a sintered or powdered carbide base matrix) during the manufacture of the bit.

The bit designer has virtually unlimited combinations of bit shape, the placement of hydraulic jetting ports, the amount of diamonds and the size of the diamonds used (usually expressed as diamonds per carat).

In general, a diamond bit that drills faster has a shorter lifetime.

Similarly, a bit designed for extremely long life will typically drill at a slower rate.

If a bit has a relatively high number of diamonds compared with other bits, it is said to be “heavy-set” and has higher durability.

A “light-set” bit, on the other hand, drills more aggressively, but wears out faster because fewer diamonds do the work.

A porous medium has a permeability of 1 darcy when differential pressure of 1 atmosphere across a sample 1 centimeter long and 1 square centimeter in cross section will force a liquid of 1 centipoise of viscosity through the sample at the rate of 1 cubic centimeter per second.

The permeability of reservoir rocks is usually so low that it is measured in millidarcys.